This invention relates to an electric noise absorber which electromagnetically shields an electric cable of an electronic device or the like and attenuates electric noise on the electric cable.
Electric noise entering an electronic device from the outside via an input electric cable and/or an output electric cable or generating from the electronic device may cause malfunctions of the electronic device or give adverse effect to other devices. To solve these problems, electric noise absorbers for electromagnetically shielding the input electric cable and/or the output electric cable are employed.
In conventional electric noise absorbers, a magnetic substance surrounding the input electric cable and/or the output electric cable absorbs and reflects electric noise transmitted along the input electric cable and/or the output electric cable, thus preventing interference by leakage of the electric noise.
Japan Published Examined Utility Model Application No. S62-14770 proposes such an electric noise absorber. A pair of ferrite pieces shaped to surround an electric cable are accommodated in a case member and the electric noise absorber is fitted onto the electric cable by means of engagement members. The ferrite pieces have to be closely connected to each other at their abutting planes in order efficiently to electromagnetically shield the electric cable. Accordingly, the electric noise absorber is constructed to press the ferrite pieces against each other due to the elasticity of the case members.
However, it is difficult to endow the case members of the electric noise absorber with two incompatible properties strength for securely holding the ferrite pieces and elasticity for sufficiently pressing the ferrite pieces. Therefore, Japan Published Unexamined Utility Model Application No. S63-39997 discloses a case member having an aperture at its bottom. As shown in FIG. 1, a biasing member 45 is integrally molded in an aperture 43 at a bottom 41 of a case member 49. A tip 47 of the biasing member 45 pushes up an underside of a ferrite piece contained in the case 49, thus applying a force to the ferrite piece.
Yet, another magnetic substance outside of the case 49 may possibly come in contact with the ferrite piece through the aperture 43. When this happens, the whole magnetic circuit of the ferrite pieces is deformed and thus the ferrite pieces less efficiently absorb and reflect the electric noise. Consequently, the electric noise may enter or leak from the electronic device.
Another conventional electric noise absorber comprises a pair of magnetic ferrite pieces, which form a rectangular plate, a cylinder, or the like when combined with each other at abutting planes. The abutting planes extend in the longitudinal direction parallel to an electric cable of an electronic device.
At the center of the abutting planes of half rectangular-plate ferrite pieces, longitudinally extending grooves are provided. The grooves are semicircular in cross section with slightly larger diameters than that of the electric cable. When closed, the ferrite pieces form a receiving hole through which the electric cable passes. In the same way, semi-cylindrical ferrite pieces form a receiving hole whose diameter is also slightly larger than that of the electric cable.
The ferrite pieces are accommodated in an openable case, and the case is closed such that the ferrite pieces surround the electric cable. Alternatively the ferrite pieces may be directly installed on the electric cable with adhesive tape. Consequently, a magnetic circuit with low impedance is formed on the closely connected ferrite pieces.
However, since ferrite is hard and difficult to be cut, the surfaces of the ferrite pieces tend to be rough and are not exactly horizontal surfaces. Therefore, sometimes the ferrite pieces may not be securely kept in close contact with each other. To solve the problem, the abutting planes of the ferrite pieces are expensively polished with diamond abrasive grains.